Phase lock evoked response audiometer

ABSTRACT

An evoked response audiometer method and apparatus in which a patient receives an auditory stimulus signal comprising a carrier frequency which is periodically amplitude modulated and frequency modulated whereby the stimulus is at least substantially frequency specific, the brain potential signals of the patient evoked by the auditory signal being sampled to determine whether phase locking to the modulated auditory signal has occurred, the auditory signal being selectively controlled to advance or delay one modulation with respect to the other modulation to cause enhancement of the evoked response to the auditory stimulus.

FIELD OF THE INVENTION

This invention relates to an improved evoked response audiometer for usein the diagnosis of deafness.

BACKGROUND OF THE INVENTION

The diagnosis of deafness at an early stage in paediatrics is importantto enable the early fitting of hearing aids and/or cochlear implants inorder to assist language development in a hearing-impaired child. It isalso important to be able to diagnose deafness in adults who are unable,due to mental illness or disability, or unwilling, for various reasons,to participate in conventional behavioural deafness testing.

In our U.S. Pat. Nos 4,462,411 (Rickards) and 5,023,783 (Cohen andRickards), we have described evoked response audiometers which use acontinuous auditory tone that is frequency or amplitude modulated, theauditory tone being presented for a sufficiently extended period of timeto enable phase-locked steady-state potentials to be evoked in the brainof the person being tested. An electro-encephalograph (EEG) signal fromthe scalp of the person is manipulated such that the components due tothe modulation carried by the auditory stimulus is extracted anddetected.

The modulated auditory stimulus produces separate and distinct evokedpotentials in the brain depending on the nature of the modulation. Theseevoked potentials can be difficult to detect, particularly for low soundlevels which are less audible to the person being tested.

SUMMARY OF THE INVENTION AND OBJECT

It is therefore an object of the present invention to provide animproved evoked response audiometer incorporating an improved modulationtechnique which produces stronger evoked potentials using low soundlevel auditory stimulus signals.

The invention provides an evoked response audiometer comprising meansfor supplying to a patient an auditory stimulus signal consisting of acarrier frequency which is modulated by at least two different forms ofmodulation such that the stimulus is at least substantially frequencyspecific, said auditory signal being presented for a sufficientlyextended period of time to enable phase-locked steady-state potentialsto be evoked in the brain of the patient, means for sampling the brainpotential signals evoked by said auditory signal, and means foranalysing said brain potentials to determine whether phase-locking ofsaid brain potentials to the modulated auditory signal has occurred,said means for supplying said auditory signal being selectivelycontrolled to advance or delay one modulation with respect to the othermodulation to cause enhancement of the evoked response to the auditorystimulus.

Research has indicated that the combined modulation of the auditorystimulus enables significant improvements in the detection of the evokedpotentials whereby evoked potentials of amplitude large enough fordetection will be produced by auditory stimuli of lower sound level, andhence lower subjective loudness.

The invention also provides a method of testing for hearing impairment,comprising the steps of supplying to the patient an auditory stimulussignal consisting of a carrier frequency which is modulated by at leasttwo different forms of modulation so that the stimulus is at leastsubstantially frequency specific, presenting the auditory signal for asufficiently extended period of time to enable phase-locked steady-statepotentials to be evoked in the brain of the patient, sampling the brainpotential signals evoked by said auditory signal, analysing said brainpotentials to determine whether phase-locking of said brain potentialsto the modulated auditory signal has occurred, and selectivelycontrolling said auditory signal to advance or delay one modulation withrespect to the other modulation to cause enhancement of the evokedresponse to the auditory stimulus.

In a preferred form of the invention, the auditory stimulus signal isamplitude modulated and frequency modulated, preferably in a periodicmanner, such as sinusoidal. The potentials evoked in the brain byamplitude modulation and frequency modulation have been found to differin phase, indicating different delays in the processing by the auditorysystem to amplitude modulation and frequency modulation. By compensatingfor the delay in perception of the amplitude and frequency modulation,the auditory signal compensates for the auditory system process byartificially advancing or retarding in time the amplitude modulation orthe frequency modulation relative to each other, resulting in theequalisation of the phase delays occurring in the evoked brainpotentials.

Without the necessary equalisation, the response to the amplitudemodulated signal and the response to the frequency modulated signal canhave a phase relationship which results in response cancellation whenthe responses are vectorially summed. By compensating for the delays inthe actual auditory stimulus, the phase of the two responses can bealtered so that the vectorial sum is significantly enhanced beyond thestimulus achieved by the use of amplitude modulation or frequencymodulation alone. This enhancement results in a higher detectionsensitivity to the stimulus by virtue of an improved signal to noiseratio, and consequently, the hearing threshold determined when using theevoked response audiometer much closer to the true behavioural hearingthreshold of the patient under test. As a result, estimations of thetrue behavioural thresholds from the patients evoked response thresholdsare improved.

Depending on the frequency of the carrier, the modulation frequency andthe corresponding modulation indexes of the auditory signal, themeasured physiological delays will vary. All such delays can becompensated for by adjusting the phase relationship between the AMmodulation and the FM modulation of the stimulus signal.

In terms of hearing perception, AM is produced by modulating a pure tone(or sinusoid) whose amplitude is varied in a sinusoidal manner byanother sine wave at the modulation frequency. FM is produced bymodulating a pure tone whose frequency is varied in a sinusoidal mannerby a sine wave at the modulation frequency. When both forms ofmodulation are combined, the frequency and amplitude can be variedtogether in a number of subtly different ways. For example, thefrequency can be high when the amplitude is high; the frequency can below when the amplitude is high; the frequency can be midway when theamplitude is high, or the frequency can be midway when the amplitude islow.

The relative phase between the AM signal and the FM signal can be givenany value between +/− about 60°, depending on the signal parameters, toproduce enhanced evoked potentials in the brain of the patient.

The responses to AM and FM stimuli, detected in the overall EEGactivity, differ. To improve the detection process both modulationmethods are used together and the phase difference between the AM and FMmodulations is selected to result in constructive addition of the AM andFM response components. In a preferred form, this occurs when the phasedifference between the AM and FM modulations is about 30°. If themodulation components are about 210° apart, cancellation will occur. TheAM/FM stimulus in this case would produce no or very little detectedresponse to the stimulus.

The phase relationship between the AM and FM detection processes dependson the mechanics of the ear and brain physiology. It also depends on themodulation indices used. The modulation indices determine how much thecarrier amplitude is changed by amplitude modulation and how much thefrequency of the carrier is changed by the frequency modulation.

It is expected that different relative phase delays will be requireddepending on the patient tested, the carrier and modulation frequencies,and the AM and FM modulation indices used. Norms for different agegroups and conscious states are determined experimentally. To thisextent the solution of the more appropriate phase difference isinitially determined empirically. However, once an appropriate phasedifference is determined, it can be used for similar patient types andsimilar signal parameters. The system may be designed to determine andbe used for diagnosis of particular hearing problems when the phasedelays used for normal patients do not provide a response as expected bythose norms. Calculations indicate a difference in the optimum AM/FMphase relationship of about 30°+/−20° would contain any detection lossto less than about 0.1 dB. If the vectors are more substantially out ofphase, a loss of up to about 9.5 dB can occur. For different signalparameters, a difference in phase of up to about +/−60° may producesimilar benefits depending on the relative amplitude of the AM and FMresponses. If the relative amplitudes are equal, a gain of up to 6 dBwill result (see FIG. 10) but if the relative amplitudes are half eachother then a loss of benefit results (see FIG. 11).

Calculations indicate that the combined modulations can result in atypical improvement in the signal to noise ratio of about 3.5 dBcompared to the response over that of AM used alone. Since the responsesbeing detected are very small compared to the background noise level,this improvement should be considered to be substantial. This assumesthe EEG voltage of an FM response is typically half the EEG voltage ofan AM response for the same stimulus level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of a signal which has been amplitude modulated (AM);

FIG. 2 is a graph of a signal which has been frequency modulated (FM);

FIGS. 3 to 6 are graphs illustrating AM/FM modulation;

FIGS. 7 and 8 illustrate the envelopes for the spectra of AM/FMmodulation for the time waveforms of FIGS. 3 and 4 respectively, withthe relative amplitudes in dB shown against frequency;

FIG. 9 illustrates the envelopes for the spectra of AM/FM modulation forthe time waveforms of FIGS. 5 and 6 respectively, with the relativeamplitudes in dB shown against frequency;

FIGS. 10 and 11 illustrate the gain or loss expected in dB when the AMand FM evoked responses are of the same amplitude and half or double theamplitude respectively compared to an AM response only;

FIGS. 12 to 15 are graphs of vector diagrams showing amplitude vs phase,illustrating responses evoked from a single subject;

FIG. 16 illustrates graphs of animal tests showing the difference inphase in radians between the auditory stimulus and the evoked responseand the frequency of the carrier at a modulation frequency of 140 Hz,and

FIG. 17 illustrates graphs of the evoked response voltages and dBrelative to one volt against the frequency of the carrier at amodulation frequency of 140 Hz.

DESCRIPTION OF PREFERRED EMBODIMENT

The evoked response audiometer embodying the invention uses digitalsignal processing (DSP) techniques for the generation of an auditorysignal or stimulus and the detection of the response to the stimulus inthe EEG activity representing the evoked potentials produced by theauditory signal. Consequently the processes used are implemented insoftware contained within the digital signal processor circuit used.This software is under direction of another software program whichresides on a personal computer (PC) using Windows95™ or a similarcomputer and/or operating system.

Conventional signal processing algorithms used in the DSP software, whenused in conjunction with each other, produce the required results.Frequency modulation and phase modulation are considered synonymous, asthe modulation used is a single sinusoidal tone. Electronic hardware ofthe type described in our earlier U.S. Pat. Nos. 4,462,411 and5,023,783, the contents of which are incorporated into the presentspecification by cross reference, is incorporated into the DSP and PCsoftware programs. Delta-Sigma analog to digital and digital to analogconverters are used to translate back and forth between the analogdomain and the digital domain. The EEG signal is amplified by means of abattery operated electronic circuit and is then transmitted using afibre optic cable to the main processing circuitry. This enhances thesafety of the patient under test.

As the embodiment is implemented by programming mathematical algorithmsinto assembly code for the DSP to execute, these algorithms express thefunction of the apparatus. The algorithms which describe the AM/FMrelative angle specifically are therefore presented in mathematicalform. The AM/FM relative angle is given the symbolic label of φ.

The computer program, under the direction of the operator, can control:

A_(c) amplitude of unmodulated carrier

m_(a) AM modulation index (0→1.0)

f_(m) modulation frequency

f_(c) carrier frequency

β the peak phase deviation or FM modulation index in radians

f_(d) the frequency deviation

φ the AM/FM relative modulation phase

The following preferred equations are implemented in the DSP softwareusing the values specified by the user listed above.

1) For signal tone AM modulation AM

 e _(AM)(t)=A _(c) [m _(a) cos(2πf _(m) t)+1] cos(2πf _(c) t)  1)

Where:

e_(AM)(t) voltage at time t

A_(c) amplitude of unmodulated carrier

m_(a) AM modulation index (0→1.0)

f_(m) modulation frequency

f_(c) carrier frequency

The increase in the signal power level in dB for a specified AMmodulation index over that of a pure sine wave output (ie unmodulatedcarrier or pure tone where the AM modulation index=0) can be calculatedas follows: $\begin{matrix}{{\Delta \quad P_{dB}} = {{10\quad {\log_{10}\left( {1 + \frac{m_{a}^{2}}{2}} \right)}} = {1.7609\quad {dB}\quad {for}\quad 100\% \quad {modulation}}}} & \left. 2 \right)\end{matrix}$

This equation still holds when FM modulation and AM modulation are usedtogether.

2) For signal tone FM modulation FM $\begin{matrix}{{e_{F\quad M}(t)} = {A_{c}{\cos \left\lbrack {{2\pi \quad f_{c}t} + {\left( \frac{f_{d}}{f_{m}} \right){\sin \left( {2\pi \quad f_{m}t} \right)}}} \right\rbrack}}} & \left. 3 \right)\end{matrix}$

 Since the frequency deviation is constant, the modulation index62=f_(d)/f_(m) varies with the modulating frequency.  4)

Where:

e_(FM)(t) voltage at time t

A_(c) amplitude of unmodulated carrier

β the peak phase deviation or modulation index in radians

f_(d) the frequency deviation

f_(m) modulation frequency

f_(c) carrier frequency

The modulation index has no effect on the output power level whichremains constant.

Also $\begin{matrix}{{e_{F\quad M}(t)} = {A_{c}{\cos \left\lbrack {{2\pi \quad f_{c}t} + {\left( \frac{m_{fm}}{2\pi \quad f_{m}} \right){\sin \left( {2\pi \quad f_{m}t} \right)}}} \right\rbrack}}} & \left. 5 \right) \\{{e_{F\quad M}(t)} = {A_{c}{\cos \left\lbrack {{2\pi \quad f_{c}t} + {\left( \frac{f_{dev}}{f_{m}} \right){\sin \left( {2\pi \quad f_{m}t} \right)}}} \right\rbrack}}} & \left. 6 \right)\end{matrix}$

 Where m _(fm)=Δω_(c)=2πf _(dev) is the peak frequency deviation  7)

$\begin{matrix}{\beta = {{\Delta \quad \theta} = {\left( \frac{\Delta \quad \omega_{c}}{2\pi \quad f_{m}} \right) = {\left( \frac{m_{fm}}{2\pi \quad f_{m}} \right) = {\left( \frac{f_{dev}}{f_{m}} \right)\quad {is}\quad {the}\quad {peak}\quad {phase}\quad {deviation}\quad {in}\quad {radian}}}}}} & \left. 8 \right)\end{matrix}$

3) For signal tone AM/FM modulation, AM/FM, and a AM/FM relative phaseangle of φ. $\begin{matrix}{{e_{{{AM}/F}\quad M}(t)} = {{A_{c}\left\lbrack {{m_{a}{\cos \left( {2\pi \quad f_{m}t} \right)}} + 1} \right\rbrack}{\cos \left\lbrack {{2\pi \quad f_{c}t} + {\left( \frac{f_{dev}}{f_{m}} \right){\sin \left( {{2\pi \quad f_{m}t} + \varphi} \right)}}} \right\rbrack}}} & \left. 9 \right)\end{matrix}$

or alternatively $\begin{matrix}{{e_{{{AM}/F}\quad M}(t)} = {{A_{c}\left\lbrack {{m_{a}{\cos \left( {{2\pi \quad f_{m}t} + \varphi} \right)}} + 1} \right\rbrack}{\cos \left\lbrack {{2\pi \quad f_{c}t} + {\left( \frac{f_{dev}}{f_{m}} \right){\sin \left( {2\pi \quad f_{m}t} \right)}}} \right\rbrack}}} & \left. 10 \right)\end{matrix}$

Where:

e_(AM/FM)(t) voltage at time t

φ AM/FM relative modulation phase

4) For signal tone PM modulation PM

e _(PM)(t)=A _(c) cos[2πf _(c) t+m _(pm) cos(2πf _(m) t)]  11)

Where

m _(pm)=Δθ the peak phase deviation  12)

5) FM and PM can be considered equal when single tone modulation isused.

ω_(PM)(i)=ω_(c) −m _(pm)ω_(m) sin ω_(m) t  13)

and

ω_(FM)(i)=ω_(c)+Δω_(c) cos ω_(m) t  14)

$\begin{matrix}{{{If}\quad \Delta \quad \omega_{c}} = {{m_{pm}\omega_{m}\quad {then}\quad m_{pm}} = {\frac{\Delta \quad \omega_{c}}{\omega_{m}} = {\beta = {\frac{f_{DEV}}{f_{m}} = {\Delta \quad \theta}}}}}} & \left. 15 \right)\end{matrix}$

So if m_(pm)=β then the modulation methods are identical except for thephase relationship between the carrier and the modulation ie SIN vs COS.

6) AM/FM Spectrum

The AM and FM equations 1) & 3) are multiplied in time or their Fourierequivalents are convolved to produce the following, keeping in mind thatthe AM & FM frequency is identical but the AM and FM phase is separatedby φ ie the relative phase between the AM and FM.

Since some software packages, such as Excel™ and Matlab™, do not provideBessel functions results for negative orders, we compensate by using thefollowing additional equation to indicate the sign of the Besselfunction for all orders, negative or positive and supplying the Besselfunction itself with the absolute order. $\begin{matrix}{{f(n)} = \begin{bmatrix}{{n < 0},\left( {- 1} \right)^{n}} \\{{n>=0},1}\end{bmatrix}} & \left. 16 \right)\end{matrix}$

and

δ(ω) is a Dirac delta function or unit impulse and using the two Fouriertransform pairs

δ(ω−ω_(x))=F{e ^(jω) _(x) ^(t)}

and

δ(ω+ω_(x))=F{e ^(−jω) _(x) ^(t)}

and where

ω_(x) is the angular frequency of x

J_(n)(x) the Bessel functions of the first kind of order n for x

F{x} is the Fourier transfrom of x

s(x) is frequency spectrum of x

* convolution symbol $\begin{matrix}{{e_{AM}(t)} = {A_{c}\left\{ {{\cos \left( {\omega_{c}t} \right)} + {\frac{m_{a}}{2}\left\lbrack {{\cos \left( {\left( {\omega_{c} - \omega_{m}} \right)t} \right)} + {\cos \left( {\left( {\omega_{c} + \omega_{m}} \right)t} \right)}} \right\rbrack}} \right\}}} & \left. 17 \right) \\{{e_{F\quad M}(t)} = {A_{c}{\sum\limits_{n = {- \infty}}^{\infty}{{f(n)}{J_{n}(\beta)}{\cos \left( {\left( {\omega_{c} + {n\quad \omega_{m}}} \right)t} \right)}}}}} & \left. 18 \right)\end{matrix}$

Convolving the AM and FM signals to find the spectrum: $\begin{matrix}{{s(\omega)} = {F\left\{ {A_{c}{\cos \left\lbrack {{\omega_{c}t} + {\left( \frac{f_{dev}}{f_{m}} \right){\sin \left( {\omega_{m}t} \right)}}} \right\rbrack}} \right\}*F\left\{ {A_{c}\left\lbrack {{m_{a}{\cos \left( {{\omega_{m}t} + \varphi} \right)}} + 1} \right\rbrack} \right\}}} & \left. 19 \right) \\{{{and}\quad {if}\quad F\quad {M(\omega)}} = {{F\left\{ {F\quad M} \right\} \quad {then}\quad F\quad {M(\omega)}} = {A_{c}{\sum\limits_{n = {- \infty}}^{\infty}{{f(n)}{J_{n}(\beta)}}}}}} & \left. 20 \right)\end{matrix}$

then $\begin{matrix}{{{s(\omega)} = {F\quad {M(\omega)}*\left\lbrack {{A_{c}{\delta (\omega)}} + {\frac{A_{c}m_{a}^{j\quad \varphi}}{2}{\delta \left( {\omega - \omega_{m}} \right)}} + {\frac{A_{c}m_{a}^{{- j}\quad \varphi}}{2}{\delta \left( {\omega + \omega_{m}} \right)}}} \right\rbrack}}{{s(\omega)} = {A_{c}\left\lbrack {{F\quad {M(\omega)}} + {\frac{m_{a}^{j\varphi}}{2}F\quad {M\left( {\omega - \omega_{m}} \right)}} + {\frac{m_{a}^{- {j\varphi}}}{2}F\quad {M\left( {\omega + \omega_{m}} \right)}}} \right\rbrack}}} & \left. 21 \right)\end{matrix}$

where e^(jφ)=cos φ+jsin φ

and therefore in the summation the three terms represent in order aspresented:

spectrum due to the carrier

spectrum due to the lower AM sideband

spectrum due to the upper AM sideband $\begin{matrix}{{e_{{{AM}/F}\quad M}(t)} = {A_{c}{\sum\limits_{n = {- \infty}}^{\infty}\left\{ {\begin{pmatrix}{{{f(n)}{J_{n}(\beta)}} +} \\{\frac{m_{a}}{2}\begin{bmatrix}\left( {{f\left( {n + 1} \right)}{J_{{n + 1}}(\beta)}\left( {{\cos \quad \varphi} + {j\quad \sin \quad \varphi}} \right)} \right) \\\left( {{f\left( {n - 1} \right)}{J_{{n - 1}}(\beta)}\left( {{\cos \quad \varphi} - {j\quad \sin \quad \varphi}} \right)} \right)\end{bmatrix}}\end{pmatrix} \cdot {\cos \left( {\left( {\omega_{c} + {n\quad \omega_{m}}} \right)t} \right)}} \right\}}}} & \left. 22 \right)\end{matrix}$

When φ is set to zero and if the AM modulation index m_(a) is set tozero then equation 22) equals equation 18) ie FM. When φ is set to zeroand if the FM modulation index β is set to zero then equation 22) equalsequation 17) ie AM since J₀(0)=1 and the remaining orders of n equal 0.If J₀(0) is then replaced with zero the result is double sidebandmodulation (DSB).

The magnitude can be found by summing the Real and Imaginary terms thentaking the square root of the sum of the squares for each Real andImaginary sum found.

In the illustrated modulations of FIGS. 3 to 6, the following featuresare present.

In FIG. 3 the carrier frequency is at its highest when the modulationfrequency amplitude is at its highest.

In FIG. 4 the carrier frequency is at its highest when the modulationfrequency amplitude is at its lowest.

In FIG. 5 the carrier frequency is at its highest when the modulationfrequency amplitude has risen to half its maximum amplitude.

In FIG. 6 the carrier frequency is at its highest when the modulationfrequency amplitude has fallen to half its maximum amplitude.

Referring to FIGS. 10 and 11, as the relative phase is changed the tworesponses either enhance (at 0 degrees) or counteract (at 180 degrees)each other. FIG. 11 is the same as FIG. 10 but the FM response is halfthe level of the AM response.

FIGS. 12 and 14 show the individual responses for AM and FM. In thesefigures the responses are approximately 180 degrees apart.

FIG. 13 shows the result when the vectors in FIGS. 12 and 14 arecombined. The individual AM and FM responses oppose each other and theAM/FM response is reduced. The reduction is approximately one half, ie aloss of about 4 to 6 dB, as indicated by the length of the vectors.

FIG. 15 shows the result when the vectors in FIGS. 12 and 14 arecombined. However in this case assume FIG. 12 has been rotated 180degrees to match the direction of FIG. 14. The rotation is brought aboutby adjusting the relative AM/FM angle of the stimulus. The individual AMand FM responses now reinforce each other and the AM/FM response isenhanced. The improvement is approximately twice, i.e. a gain of about 4to 6 dB, as indicated by the length of the vectors.

FIGS. 13 and 15 actually indicate that there is some small phasemisalignment in the enhancement or cancellation of the vectors oralternatively the AM and FM responses are not of equal amplitude.Consequently the deep null shown in figure ten is not achieved when thevectors are opposing as the vector diagram FIG. 13 does show a response,albeit small one. FIG. 11 is perhaps more representative of what isbeing achieved given the four vector diagrams presented.

FIG. 16 illustrates data collected from Greyhound dogs, whileanaesthetised, and shows the phase recorded from subject four using astimulus level of 50 dB HL at carrier frequencies of 500, lk, 2 k and 4KHz using different modulation types. The results for four types ofmodulation include:

The phase response using only amplitude modulation at 0°

The phase response using only frequency modulation at 0°

The phase response using AM/FM with a relative phase of 0°

The phase response using AM/FM with a relative phase of 180°

In the results labelled as “calc”, the recorded data from the AM and FMonly tests were combined vectorially to see if the actual recorded AM/FMtests with relative phases of 0 and 180° could be duplicated bycalculation alone. The calculated values match the recorded values well.It was found the AM signal needed to be in the range 1.2 to 1.4 timesthe FM signal voltage level to match the AM/FM recorded data.

The recorded AM/FM response with a relative phase of 0° is a moreprecise match with the calculated values than the recorded AM/FMresponse with a relative phase of 180°. The 180° relative phaseresponses are less accurate as the cancellation effect diminishes theamplitude of the response compared to the background EEG noise, ie. thesignal to noise ratio diminishes. Under these circumstances the measuredphase is more prone to error.

The line marked “AM—Pi ref” is the AM result with a relative angle of 0°shifted 180° and is used as a reference line. From 500 Hz to 2000 Hz,the AM and FM signals are very close to being in phase. Therefore theAM/FM result for these carrier frequencies, using a relative phase of180°, should intersect the reference line, as it does.

FIG. 17 shows the voltage of the EEG signal in dB referenced to oneVolt. At carrier frequencies 500 to 2000 Hz where the AM and FM signalsare very close to being in phase, we find that the combined signalvoltage using AM/FM with a relative phase of 0° is enhanced over that ofAM or FM alone. Conversely using AM/FM with a relative phase of 180° thesignal level is considerably reduced due to cancellation. It followsfrom these results that relative phases other than 0° will result infurther enhanced evoked potentials at the same or different stimulussignal parameters.

While the preferred modulation modes are AM and FM, other continuousmodulation modes may be able to be used with acceptable results.

It will also be appreciated that various modifications and/oralterations may be made to the system described above without departingfrom the scope and spirit of the invention.

We claim:
 1. An evoked response audiometer comprising means for supplying to a patient an auditory stimulus signal consisting of a carrier frequency which is modulated by at least two different forms of modulation such that the stimulus is at least substantially frequency specific, said auditory signal being presented for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the patient, means for sampling the brain potential signals evoked by said auditory signal, and means for analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, said means for supplying said auditory signal being selectively controlled to advance or delay one modulation with respect to the other modulation to cause enhancement of the evoked response to the auditory stimulus.
 2. The audiometer of claim 1, wherein the auditory signal is amplitude modulated and frequency modulated.
 3. The audiometer of claim 2, wherein the auditory signal is modulated in a periodic manner.
 4. The audiometer of claim 1, wherein the means for supplying said auditory signal is selectively controlled so that there is a difference in phase of up to about +/−60°.
 5. The audiometer of claim 4, wherein the difference in phase is about +/−30°+/−20°.
 6. A method of testing for hearing impairment, comprising the steps of supplying to the patient an auditory stimulus signal consisting of a carrier frequency which is modulated by at least two different forms of modulation such that the stimulus is at least substantially frequency specific, presenting the auditory signal for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the patient, sampling the brain potential signals evoked by said auditory signal, analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, and selectively controlling said auditory signal to advance or delay one modulation with respect to the other modulation to cause enhancement of the evoked response to the auditory stimulus.
 7. The audiometer of claim 6, wherein the auditory signal is amplitude modulated and frequency modulated.
 8. The audiometer of claim 7, wherein the auditory signal is modulated in a periodic manner.
 9. The audiometer of claim 6, wherein the means for supplying said auditory signal is selectively controlled so that there is a difference in phase of up to about +/−60°.
 10. The audiometer of claim 9, wherein the difference in phase is about +/−30°+/−20°.
 11. The audiometer of claim 2, wherein the means for supplying said auditory signal is selectively controlled so that there is a difference in phase of up to about ±60°.
 12. The audiometer of claim 3, wherein the means for supplying said auditory signal is selectively controlled so that there is a difference in phase of up to about ±60°.
 13. The audiometer of claim 7, wherein the means for supplying said auditory signal is selectively controlled so that there is a difference in phase of up to about ±60°.
 14. The audiometer of claim 8, wherein the means for supplying said auditory signal is selectively controlled so that there is a difference in phase of up to about ±60°. 